In high frequency communication systems typified by mobile phones, high-frequency filters have been used to remove unnecessary signals other than signals in the frequency band used for communication. The high-frequency filter includes an acoustic wave device having a surface acoustic wave (SAW) element or the like. The SAW element is an element in which an Interdigital Transducer (IDT) having a pair of comb-shaped electrodes is formed on a piezoelectric substrate. It has been known that the loss is reduced by making the acoustic velocity of the surface acoustic wave excited by the IDT lower than the acoustic velocity of a bulk wave propagating through the piezoelectric substrate, as disclosed in, for example,
Japanese Patent Application Publication No. 2016-136712 (hereinafter, referred to as Patent Document 1).
It has been known that when the IDT is formed of an Al film, an amorphous Al underlayer or an Al underlayer having a fine-grained structure is formed and then an Al film is formed on the underlayer, as disclosed in, for example, Japanese Patent Application Publication No. H05-226337 (hereinafter, referred to as Patent Document 2). It has been known to use a Pt film, a Mo film, or a W film as the IDT, as disclosed in, for example, Japanese Patent Application Publication No. 2015-73331 (hereinafter, referred to as Patent Document 3).